Salt bath



Patented Sept. 7, 1943 SALT BATH Johann S. Streicher, Newark, N. J.,assignor to The American Platinum Works, Newark, N. J a corporation ofNew Jersey No Drawing. Application November 25, 1941,

Serial No. 420,342

2 Claims.

. This invention relates to salt bath furnaces and is directedparticularly to an improvement in salt baths for salt bath furnaces andthe annealing of metals therein.

Salt bath furnaces are used primarily. for the annealing of metals atelevated temperatures. Salt baths as heretofore known usually comprisemixtures embodying one or more of such ingredients as sodium chloride,potassium chloride, soda ash, potash, sodium nitrates, potassiumnitrates, cyanides, etc. One well known salt bath comprises, forinstance, a mixture of 50% potassium chloride and 5Q% sodium carbonate.Such salt baths produce, however, an alkaline reaction and tend tocorrode metals to be annealed and to produce surface conditions on suchmetals which are detrimental to subsequent soldering or weldingoperations. Salt baths embodying cyanides produce highly, poisonousfumes and salt baths embodying nitrates and nitrites are highlyoxidizing and have only invention to provide a.

develop poisonous fumes in use and which shall not be of oxidizingnature. It is another object of gmy invention to anneal metals in suchsalt bat i I have found that an excellent salt bath-for salt bathfurnaces is produced by an aqueous mixture of boric acid 'and sodiumfluoride in the proportions of 55% to 71% boric acid and 45% to 29%sodium fluoride. the mixture which'is exothermic, evolving considerableheat, and produces ahydrated polyboric acid compound, probablytetraboric acid and pentaboric acid, containing water as a constituentthereof. -The compound alsoseems to contain sodium fluoride as aconstituent there-- of. When heated "the reacted mixture quickly losesany excess of water, but the water which is A reaction occurs in 40 notonly metals as such but also alloys and other contained in the boricacid compoundas a conmass has a. high liquidity and fluidity, a factorof great importance in producing a successful operative salt bath forsalt bath furnaces. Likewise, such mass has a low surface tension. The

lidify as a transparent glassy mass and do not.

have a definite melting point.

In the salt bath according to my invention there is produced a weak acidreaction, rather than an alkaline reaction as heretofore, and nocorrosion of metals takes place during the annealing. The liquid saltbath according to my invention also has an excellent cleaning effectupon the material annealed therein. Harmful alkaline films are entirely.eliminated and no poisonous fumes are developed. In view of the highliquidity and fluidity of my salt bath mechanical losses in using suchbath are kept at a.

minimum. Oxidation or other contamination of the object being annealedis prevented due to the nature of my salt bath composition.

My. salt baths can beutilized in the annealing of ferrous as well asnon-ferrous metals and metal compositions and-is suitable for theannealing of brass, copper, aluminum, steel, stainless steel, and othermetals of base metal nature as well as in the annealing of preciousmetals such as silver, gold, platinum, etc. The object to be annealed inmy salt baths may be in the form of sheet, wire or any other suitableshape; The

saltbath furnace, containing the molten salt bath according to thisinvention, may be-operated electrically or-may be fuel fired.

The term metal? as ,herein used shall, include metal compositions orcombinations, and the term annealing shall also include any other formof heat treatment.

The hydrated complex polyboric acid compound salt bath is best producedby'flrst mixing together, preferably in a mortar or grinding machine,the sodium ,fluoride'and boric acid in the specified proportions andthen. adding the proper amount of water. Thus about 35 to" cc. water maybe added to 100 grams mixture according to my invention of sodiumfluoride and boric acid to result in the desired hydration. Since boricacid -does not readily wet water, there may be added to. the water atrace of wetting agent. The

liquid crystallizes while evolving heat to result in a hard mass whichis then ground into a thin paste. The mass may also be produced bydissolving sodium fluoride in hot water and then adding boric acid, thehydrated complex polyboric acid compound crystallizing after cooling.For

instance, a mixture of commercial boric acid,

dehydrated commercial sodium fluoride, and 750 cc. water containing atrace of Lamepon A as wetting agent, is finely. ground until the mass istransformed into a thick gray liquid which on standing turns itself bycrystallization into a white mass which is thenthoroughly ground andthereby transformed into a smooth paste.

The salt bath for salt bath furnaces, according to my invention, mayalso be produced by ing said metal in a molten salt bath consisting ofthe reaction product of a mixture of 55% to 71% boric acid and 29% to45% sodium fluoride.

JOHANN S. STREICHER.

